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Description

Sunrise-Sunset experiment

This experiment shows how thicker substances change the way light shines through them and therefore changes the colour of the light out the other side.

Resources

clear drinking glass/small glass baking dish
water
whole milk (not trim/light)
spoon to mix milk and water together
torch
Dark place to more clearly show light shining through the glass. Doesn't need to be fully dark.

Instructions

Activity:
Half fill a clear drinking glass with water.
Shine your torch on the glass directly above it.
The water will look white.
Now pour milk into the glass of water to fill and mix well.
Take the glass somewhere dark and shine the torch from the side of glass through the milk.
The milk will look yellow, orange or red.

Why?
Just like a sunset or sunrise, the light is shining through particles.
The atmosphere is full of billions of particles like the gases that make up the air, dust, water vapour (clouds).
When the sun is low in the morning or early evening, it’s rays must travel through a thicker layer of atmosphere than at other times of the day.
When sunlight hits these particles, the light bounces off of them and scatters. Orange and red lights scatter the least, which is why you see these colours in a sunrise or sunset.

Why? Extension explanation
Particles from the milk are scattering light. Examine the container from all sides. Notice if you look at the container from the side, the torch beam looks slightly blue, while the end of the flashlight appears slightly yellow.
How It Works
Light travels in a straight line until it encounters particles, which deflect or scatter it. In pure air or water, you can't see a beam of light and it travels along a straight path. When there are particles in the air or water, like dust, ash, ice, or water droplets, light is scattered by the edges of the particles.

Milk is a colloid, which contains tiny particles of fat and protein. Mixed with water, the particles scatter light much as dust scatters light in the atmosphere. Light is scattered differently, depending on its color or wavelength. Blue light is scattered the most, while the orange and red light is scattered the least. Looking at the daytime sky is like viewing a flashlight beam from the side -- you see the scattered blue light. Looking at sunrise or sunset is like looking directly into the beam of the flashlight -- you see the light that isn't scattered, which is orange and red.

What makes sunrise and sunset different from the daytime sky? It's the amount of atmosphere the sunlight has to cross before it reaches your eyes. If you think of the atmosphere as a coating covering the Earth, sunlight at noon passes through the thinnest part of the coating (which has the least number of particles). Sunlight at sunrise and sunset has to take a sideways path to the same point, through a lot more "coating", which means there are a lot more particles that can scatter light.

While multiple types of scattering occur in the Earth's atmosphere, Rayleigh scattering is primarily responsible for the blue of the daytime sky and reddish hue of the rising and setting sun. The Tyndall effect also comes into play, but it is not the cause of blue sky color because molecules in air are smaller than the wavelengths of visible light.

Extension to Activity:
Stir more milk into the water. As you increase the number of particles in the water, the light from the torch is more strongly scattered. The beam appears even bluer, while the path of the beam furthest from the torch goes from yellow to orange. If you look into the torch from across the g;ass, it looks like it is orange or red, rather than white. The beam also appears to spread out as it crosses the container. The blue end, where there are some particles scattering light, is like the sky on a clear day. The orange end is like the sky near sunrise or sunset.

Tags

• Red sunset
• Sunrise
• Sunset
• Weather Wise

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